Boa constrictors stop their victims’ hearts
The snakes kill by cutting off blood flow, not breath
Boa constrictors don’t suffocate their prey so much as break their hearts. A new study finds that these snakes kill like demon blood-pressure cuffs. They squeeze down blood circulation until it stops. Without blood delivering fresh oxygen, the heart, and brain starve.
This means that the idea boa constrictors slay by preventing their prey from breathing is just plain wrong.
Scott Boback emphasizes to his students at Dickinson College in Carlisle, Pa., that these snakes are extraordinary hunters. They don’t need limbs, or even venom, to bring down an animal their own size. “Imagine,” he says, “you’re killing and swallowing a 68-kilogram [150-pound] animal in one meal — with no hands or legs!” As an animal ecologist, Boback studies how different creatures relate to each other. The question of how boa constrictors kill their prey is one example.
For their hunting to succeed, the speed at which these snakes kill really matters. Keep in mind that their prey can fight with claws, hooves or other weapons that the snake lacks. Embracing a creature into heart failure is faster than suffocating it, Boback points out.
The boa is the iconic constrictor. It’s home range spans from Mexico south to Argentina in South America. This snake ambushes birds, monkeys and a wide range of other animals. It launches its attacks in much the same way each time. The snake cinches a loop or two around its victim’s upper body. Then it presses hard enough to starve organs of oxygen-rich blood.
“It’s not some unbelievable amount of pressure,” says Boback. His arms get snaked by a boa now and then. “It stings a little,” he says. “You can kind of feel the blood stop.”
Boback and his colleagues have studied boas’ deadly grip in the laboratory. They use anesthetized lab rats as prey. Within six seconds of looping around a rat, a boa constrictor squeezes enough to cut the blood pressure in a rear-leg artery in half. Blood that should surge through the artery will now lie dammed behind snake coils in the rat’s upper body. And back pressure keeps the rat heart from pumping out new blood. Circulation falters. Eventually, it fails altogether. The boas then release their grip after about six minutes, on average. That’s when they sense their prey are dead.
Boback and his colleagues described their new findings July 15 in the Journal of Experimental Biology.
Once a boa’s prey is dead, the snake swallows it whole. A rat about a quarter of the snake’s weight disappears down the boa’s gullet in a couple of minutes. Moveable bones in the head help the snake open wide enough to let in the meal. So does a dimple of stretchy cartilage that lets the chin open really wide.
People often tell Boback that snake jaws must separate at the back. But that, he notes, is just another serpentine myth.